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Home » Software » SpectraFoo » How do I safely measure a power transformer using a signal generator and scope?
How do I safely measure a power transformer using a signal generator and scope? [message #86637] Fri, 12 January 2018 03:42 Go to next message
Repusheeran is currently offline  Repusheeran
Messages: 1
Registered: January 2018
Junior Member
I have a power transformer that I pulled out of a UPS a couple of years ago and I'd like to reuse it in a linear power supply.

It has 2 wires on one side and 3 on the other. Currently, I don't know which side is primary and which one is secondary but I suspect, maybe wrongly, that the side with 2 wires is primary. The side with the two wires has screw terminals (a ring to put a screw through) and the other side has plug terminals that if I remember well used to plug to the board.

index.php?t=getfile&id=134&private=0

Now, since I'm not sure about a lot of things about this transformer, I don't want to just wire it up and measure it while connected in the mains, especially since I'm not very experienced with mains voltages. (I do work servicing 3-phase dimmers but it's different when you have the safety measures we have at work).

My thought was to connect it to a signal generator and scope (I have a USB combo) and see if the other side is of higher or lower voltage in order to identify the primary and secondary sides. Since this was part of the power supply of a UPS, in theory the primary should take in 230V and the secondary should give out a voltage that when rectified would be sufficient to charge 12V batteries + maybe a second secondary for +5V logic(?). So I estimate that I expect a 20something:1 drop in voltage on one secondary and a 50ish:1 on the other.

To formalize this question a bit:

How do I safely measure a power transformer using a signal generator and scope without damaging my equipment?

Is a 1V (peak to peak), 50Hz sine wave signal sufficient/suitable for the measurement?

I suspect I need to measure the secondaries across a load. Will a resistor around 1MΩ (1/4W) be sufficient (or necessary)?

Finally, please let me know if I'm moving in totally the wrong direction.
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Re: How do I safely measure a power transformer using a signal generator and scope? [message #87434 is a reply to message #86637] Wed, 18 December 2019 02:22 Go to previous message
ericaturner is currently offline  ericaturner
Messages: 1
Registered: December 2019
Location: USA
Junior Member
First of all, I wanna say I loved your writing. From beginning to end you nailed it totally. To write this you might have worked hard for research.
Thanks for sharing this good article.
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